IPhone 7 - sudden ‘cellular update error’ and SIM card

Hi there - help pls. I’ve exhausted my tech patience and feel like I’m in an illogical twilight zone.

4 days ago my trusty iPhone 7 (carrier T-Mobile. IOS 14.2) suddenly flashed a Cellular Update error. Upper left corner displays ‘.....Searching’ + can’t connect except to WiFi. No way to access or update cell function.

I can text but not make/get calls

I read the long, informative thread about similar iOS 14 update issues.

My query: After doing all the toggles/resets etc. I removed the sim card + put my boyfriend’s in.

What?! Suddenly the phone looked, worked perfectly - except showed his Carrier.

Bought myself a new sim. Inserted and still have “.....searching” + no call use

The phone was my daughter’s, originally bought w an ATT contract. Ive used it with T-Mobile over a year.

Would the sim I just bought not work bc her original sim was ATT?

Apology for the length - thanks!